EARLY DEVELOPMENT
&
GERM LAYERS
Dr. Saleem Shaikh
Introduction
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Development starts once
the ovum and sperm fuse
to each other to form
Zygote
It goes into multiple
rounds of cell division
(cleavage) 2,4,6, and 16
cell stage where its called
as morula.
takes about 4 days for
the zygote to reach the
uterus and till then this
division continues.
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As the cleavage proceeds to 16 cell stage it is known as
morula.
If we cut a section through the morula – it shows an inner cell
mass which is surrounded by an outer layer of cells.
The outer layer of cells give rise to trophblast
Inner cell mass gives rise to embryo proper hence called as
embryoblast
Some fluid enters into the morula and seperates the cells of the
inner cell mass from the trophoblast.
Throphoblast cells flattened and inner cell mass gets attached
to one end of trophoblast
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The MORULA becomes a BLASTOCYST
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The cavity seen inside is called as blastocoele
Formation of Germ layers
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The blastocyst gives rise to all the tissues and organs of the
body, it also forms many structures which support the embryo
and help it to acquire nutrition.
This blastocyst converts to a three layered disc…known as the
embryonic disc.
The three layers are
Endoderm
 Ectoderm
 Mesoderm
All the tissues of the body are derived from these three layers
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Some cells of the inner cell mass become flattened and line the
lower surface this is known as HYPOBLAST
The remaining cells become columnar and called as EPIBLAST
A space is seen between the epiblast and the trophoblast –
this is known as amniotic cavity, filled with amniotic fluid
The hypoblast cells spread and line inside the blastocystic
cavity to form the primary yolk sac.
The cells of the trophoblast proliferate to give rise to a mass
of cells known as extra embryonic mesoderm.
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Chorion, amnion and extra embryonic coelom are formed.
The primary yolk sac becomes smaller and is known as
secondary yolk sac, and the cells become cubical
The cubical cells of the hypoblast become tall (columnar) near
the margin of the disc. This area is called as the prochordal
plate.
This helps to identify the head end of the embryo
Next some of the epiblast cells near the tail end proliferate
and bulge into the amniotic cavity. This elevation is called as
PRIMITIVE STREAK
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The cells of the primitive streak proliferate and push
themselves between the epiblast and hypoblast. These cells
form the intra embryonic mesoderm.
Some of these proliferating cells displace the hypoblast and
form the endoderm.
The remaining cells of the epiblast form the ectoderm
This process of formation of the primitive streak,
intraembryonic mesoderm and endoderm is called as
Gastrulation.
The cells of the primitive streak do not spread to the area of
the prochordal plate. Hence no mesoderm is seen in this region
and forms the bucco-pharyngeal membrane
Thank you!!!